First, note that the project was cancelled before the first version shipped. This would have changed if it had made it all the way to release.
Second, some details about the store... "Windows Store apps" can use the IE rendering engine to browse the web, but can't use other existing libraries like Gecko or Blink. "Metro-enabled desktop browsers" like Firefox and Chrome and IE (the terminology comes from an old Microsoft white paper [1]) can take advantage of a special exception to the restrictions placed on most "Metro" apps, but only if they are set as the default browser for both desktop and Metro mode. And they can't be installed like Windows Store apps; you have to install them through the desktop.
Metro-enabled desktop browsers (like other desktop apps) can be listed in the Windows Store, but can't be installed there. Instead, the Windows Store page will just contain a link an installer on the developer's web site. Chrome links to their installer from the Windows Store, and we'd have done the same if we shipped the Metro app in a Firefox release. (We could still list desktop Firefox in the Windows Store, subject to the same restrictions.)
Second, some details about the store... "Windows Store apps" can use the IE rendering engine to browse the web, but can't use other existing libraries like Gecko or Blink. "Metro-enabled desktop browsers" like Firefox and Chrome and IE (the terminology comes from an old Microsoft white paper [1]) can take advantage of a special exception to the restrictions placed on most "Metro" apps, but only if they are set as the default browser for both desktop and Metro mode. And they can't be installed like Windows Store apps; you have to install them through the desktop.
Metro-enabled desktop browsers (like other desktop apps) can be listed in the Windows Store, but can't be installed there. Instead, the Windows Store page will just contain a link an installer on the developer's web site. Chrome links to their installer from the Windows Store, and we'd have done the same if we shipped the Metro app in a Firefox release. (We could still list desktop Firefox in the Windows Store, subject to the same restrictions.)
[1] http://www.zdnet.com/firefox-for-windows-8-adds-a-touch-of-m...